fix(build): make graph construction invariant under input FASTA order

[mmolari]

pangraph build gave a different graph depending only on the order the input FASTA files were listed.

Block ids derive from input position, are passed to minimap2 as sequence names, and -X (MM_F_NO_DUAL) picks the direction of each pair by strcmp on those names. Argument order thus
decided query vs reference for every pair, and assign_anchor_block privileged the reference, so its consensus won and propagated into the merged block. Independently, neighbor
joining resolved Q-matrix ties by row order — which for three taxa is always tied, since every off-diagonal entry is identically -(D₁₂+D₁₃+D₂₃) — so the inferred topology also
followed the command line.

Changes

• align/block_names.rs: new BlockNames, naming and ordering blocks by (xxhash64(consensus), block_id) as zero-padded {hash}_{id}, so the aligner input is a pure function of block
  content. Both backends feed blocks in this order and recover ids by lookup.
• Tie-breaks in filter_matches and assign_anchor_block now use that content key instead of vector order / reference-first.
• Neighbor joining sorts its leaves by a content-derived key.
• par_bridge() → indexed par_iter().zip(), which preserves order by construction.
• tests/itest_build_order_invariance.rs: invariance over six argument permutations under three topologies, plus guide-tree sibling swap.

Guarantee: graph structure is a pure function of the input sequence set, the tree topology and the alignment parameters. BlockId values still derive from fasta.index, so output JSON
is not byte-identical across orders — only the structure is.

Validation

All six permutations of the reproducer now give 512 blocks (was 452–509) with byte-identical consensus and depth multisets, and the same under a sibling-swapped tree. Without
--guide-tree they likewise collapse to one answer. Each test assertion was checked against reverted code. Fixtures (data/test_graph.json, plasmids.json) are unchanged; tests and
clippy are clean.

Full investigation, measurements and rejected alternatives: notes/n00_build_order_dependence.typ.